Rep. Doug Lamborn (R-Colo.) is facing the toughest primary of his House career today.

His challenger, wealthy businessman Robert Blaha, has outspent Lamborn in the race for the 5th district over the course of this year. Running as an “outsider," Blaha charges that while Lamborn has been a reliable conservative, Blaha would be a more effective Member.

Blaha, who has contributed more than $700,000 in personal funds to his campaign, spent nearly $400,000 in the first quarter while Lamborn spent about $25,000. But Lamborn responded in kind in recent months, narrowly outspending Blaha in the pre-primary period, according to Federal Election Commission reports. Lamborn has focused much of his spending in the lead-up to the primary on negative television advertising.

On top of the spending issue, another problem for Lamborn is that in his previous races, he has either won the primary unopposed or with a plurality in a multicandidate field. This time around, it's a two-man race and those backing Blaha hope that he can prevail since the anti-Lamborn vote won't be split.

One sign of strength for Lamborn, however, is that the Club for Growth has not gotten involved. The group has backed him in the past but has not spent money in this primary. A lack of involvement often indicates the club’s confidence in a candidate.

This is a mail-in ballot election. All votes must be received this evening or they will not count, with the exception of overseas ballots.

This is a Safe Republican seat, so whoever wins this primary will be heavily favored in November.